1. Field of the Invention
The present invention relates to a method for preparing a functional particulate organic material for use in toners used for developing an electrostatic latent image formed by an image forming method such as electrophotography, electrostatic recording and electrostatic printing; paints, colorants, fluidity improving agents, spacers, preservation stabilizers, cosmetics, and fluorescent labels. In addition, the present invention also relates to a toner using the functional particulate organic material. Further, the present invention relates to an image forming method and an image forming apparatus (including a process cartridge) using the toner.
2. Discussion of the Background
Particulate organic materials have been used for various fields. For example, particulate organic materials can be used as toners and developers for use in electrophotographic image forming fields. In addition, particulate organic materials can also be used as fluidity improving agents, charge controlling agents, carriers and photoconductive powders, and intermediate materials therefor.
Electrophotographic developer is used for image forming methods such as electrophotography, electrostatic recording and electrostatic printing, which typically include the following processes:    (1) an electrostatic latent image formed on an image bearing member such as photoreceptors is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (2) the toner image is transferred on a receiving material such as receiving papers (transfer process); and    (3) the toner image is fixed on the receiving material upon application of heat and/or pressure, or the like (fixing process).
Dry developers are broadly classified into two-component developers which are typically constituted of a dry toner and a carrier, and magnetic or non-magnetic one-component developers which are typically constituted of a toner and which do not include a carrier.
Electrophotographic dry toners for which particulate organic materials are used are typically prepared by the following manufacturing method:    (1) a toner constituent mixture including a colorant, a binder resin (e.g., styrene resins and polyester resins) and optional additive is kneaded upon application of heat thereto (kneading process); and    (2) after being cooled, the kneaded mixture is pulverized to prepare toner particles.
It is attempted to decrease the particle diameter of toner in order to produce high quality toner images. The toner particles prepared by the pulverization method mentioned above have irregular forms, and therefore the toner particles can be further pulverized in image forming apparatus due to stresses applied to the toner particles by developing rollers, toner supplying rollers, toner layer thickness controlling blades and frictional charge applying blades of the image forming apparatus. As a result, super fine toner particles are produced and/or a fluidity improving agent located on the surface of the toner particles is embedded into the toner particles, resulting in deterioration of image qualities.
In addition, since the pulverized toners have irregular forms, the toners have poor fluidity and therefore a large amount of fluidity improving agent has to be included therein. Further, the toners have low packing ability (i.e., the amount of a toner contained in a container is relatively small), and thereby the toner bottle becomes large in size. Therefore, it becomes difficult to design a compact image forming apparatus. Namely, the advantage of the toner (i.e., small particle diameter) is not effectively exploited. Further, when a toner is prepared by a pulverization method, the particle diameter of the toner is limited (namely a toner having a very small particle diameter cannot be produced by a pulverization method).
Recently, color images are popularly produced in offices. Color image forming apparatus have a complex structure and use a complex image transfer device because plural toner images have to be transferred on proper positions of a receiving material. When a pulverized toner is used for such color image forming apparatus, a problem such that the transferred toner images have omissions due to poor transferability of the toner used occurs. In attempting to avoid this problem by increasing the amount of toner adhered to the electrostatic latent images, another problem in that the toner consumption increases occurs.
Therefore a need exists for enhancement of toner image transfer efficiency, which results in production of high quality images and reduction of toner consumption (i.e., reduction of running costs). When a toner having an excellent transfer efficiency is used, it becomes unnecessary to use a cleaning device, and thereby the image forming apparatus can be miniaturized and the manufacturing costs of the apparatus can be reduced. In addition, the image forming apparatus produces no waste toner. In attempting to solve the problems specific to the toners having irregular forms, various spherical toners and various methods for producing spherical toners have been proposed.
For example, suspension polymerization methods and emulsion polymerization/aggregation methods in which particles are prepared by emulsion polymerization, followed by aggregation of the emulsified particles have been investigated. In addition, polymer solution emulsifying methods which utilize a technique of reducing the volume of toner particles have been proposed. Specifically, the methods include the following steps:    (1) a toner constituent is dissolved or dispersed in a volatile solvent such as organic solvents having a low boiling point;    (2) the solution or dispersion is dispersed in an aqueous medium including a dispersant to form an emulsion; and    (3) the volatile solvent is removed from the emulsion to prepare a dispersion including toner particles.
This method is disclosed in, for example, published unexamined Japanese Patent Application No. (hereinafter JP-A) 07-152202.
This method has the following advantages over the suspension polymerization methods and emulsion polymerization/aggregation methods:    (1) a variety of resins can be used as the binder resin of the toner; and    (2) particularly, polyester resins which are suitable for toners for full color image forming because the resins have good transparency and the resultant toner images have smooth surface can be used as the binder resin.
However, the method has a drawback in that the dispersant used strongly adheres to the surface of the resultant toner particles to such an extent as not to be removed therefrom even when the toner particles are subjected to a washing treatment, and thereby the charge properties of the toner greatly depend on the properties of the dispersant used. Namely, the resultant toner particles have low charge quantity and low charge rising speed, while the charge properties seriously change depending on the environmental humidity.
A modified polymer solution emulsion method is disclosed in JP-A 11-149179 in which a low molecular weight resin is used to reduce the viscosity of the polymer solution or dispersion, to easily perform the emulsification, and the low molecular weight resin is then polymerized in the particles of the emulsion to improve the fixability of the resultant toner. In this method, the functional groups of the resin to be polymerized and the groups of a compound to be reacted with the resin largely influence the charge properties of the resultant toner particles. In particular, when an isocyanate compound is used to be reacted with the resin, the charge properties of the resultant toner change depending on the charge properties of the resultant urea bonding or urethane bonding of the reaction product (i.e., the polymerized resin).
In order to modify the property of the surface of a particulate organic material, mechanical methods such as hybridization and mechano-fusion methods, chemical methods using a coupling agent such as silane coupling agents and titanium coupling agents and the methods disclosed in JP-As 2001-343786 and 11-84726 have been proposed.
The mechanical methods such as hybridization and mechano-fusion methods can produce considerable modification effect, but the particulate organic material to be treated receives large impact force and heat energy. In general, particulate organic materials cause a morphologic alteration. Therefore, when such mechanical methods are used, the desired function can be imparted to the material but other properties of the resultant toner tend to seriously change. Specifically, when the impact force and heat energy applied to the toner particles is reduced so that the material does not cause morphologic alteration, the effect of the surface modification is weakened. In contrast, when the impact force and heat energy is increased to sufficiently perform the surface modification, the organic material causes morphologic alteration. In addition, the apparatus used for the mechanical methods are large in size and expensive, and thereby the manufacturing costs of the toner are increased.
The chemical surface modification methods typically use a coupling agent such as silane coupling agents and titanium coupling agents. JP-As 2001-343786 and 11-84726 have disclosed such chemical methods. However, it is hard for the methods to impart a desired property to a particulate organic material. In particular, there are narrow options for the coupling agents.
Specifically, JP-A 2001-343786 discloses the following method:    (1) a metal compound of an aromatic oxycarboxylic acid, a colorant, a material having a low softening point and a polar resin are dispersed in a monomer;    (2) the mixture is polymerized in an aqueous medium to produce a particulate organic material while the pH of the system is controlled so as to be from 4.5 to 9.0 by adding a metal compound of an aromatic oxycarboxylic acid which can be dissolved in an aqueous alkali solution with pH of from 9 to 13 (first polymerization process);    (3) the polymerization is continued while the pH of the system is adjusted so as to be 9 to 13 (second polymerization process); and    (4) the reaction product in the dispersion is treated with an acid treatment using an acid with pH of from 1.0 to 2.5 to deposit the metal compound of the aromatic oxycarboxylic acid on the surface of the particulate organic material.
However, the metal compound is present on the surface of the resultant particulate material while released from the surface, namely, the surface modifying agent is not fixed on the surface of the particulate material.
JP-A 11-84726 discloses the following surface modification method:    (1) an aqueous solution of boric acid or a metal salt thereof is added to a coagulated emulsion including a colorant at a temperature in the range of from about 30 to about 95° C.; and    (2) the pH of the resultant reaction mixture is controlled so as to be from about 9 to 12 by adding a base followed by addition of salicylic acid or catechol thereto to chemically modify the surface of aggregated particles of the emulsion.
However, only zinc is exemplified as the metal of the metal salt in JP-A 11-84726, and the reaction temperature is relatively high (85° C.). Since the zinc of zinc sulfate described in JP-A 11-84726 is divalent, the zinc ion makes a coordinate bond while having four coordinate valence. Therefore, only one molecule of salicylic acid or catechol can be bonded to the zinc ion. As a result of the present inventors' study, it is found that when a divalent metal such as Zn is used, i.e., only one molecule of an organic acid is bonded thereto, the surface modification effect cannot be produced. In addition, since salicylic acid is added to the reaction mixture at an alkali region (i.e., at a pH of from 9 to 12), the reaction has to be performed at a high temperature in the range of from 30 to 95° C. In addition, the pH is maintained until the reaction is completed, and thereby a problem in that the metal compound is not perfectly reacted occurs. The reaction is performed at a high temperature (85° C.) in Example in JP-A 11-84726, the reaction product causes serious morphologic alteration, which is a big problem.
Namely, when this technique is applied to a toner having a low glass transition temperature to improve the low temperature fixability, a problem which occurs is that it becomes impossible to perform the reaction or it takes long time until the reaction is completed if the reaction temperature is relatively low.
Because of these reasons, a need exists for a simple surface treatment method by which a variety of surface modifying agents can be firmly fixed on the surface of organic particles to impart a desired function to the particles without causing problems such as morphologic alteration due to heat and mechanical shock.